The present invention comprises a process for the in-situ precipitation of ceramic material in a metallic matrix, wherein the ceramic comprises a boride, carbide, oxide, nitride, silicide, etc., of one or more metals other than the matrix metal. The matrix metal, moreover, may constitute an alloy of two or more metals.
For the past several years, extensive research has been devoted to the development of metal matrix composites, such as aluminum reinforced with carbon, boron, silicon carbide, silica, or alumina fibers, whiskers, or particles. Metal matrix composites with excellent high temperature yield strengths and creep resistance have been fabricated by the dispersion of very fine (less than 0.1 micron) oxide or carbide particles throughout the metal or alloy matrix. The production of such dispersion-strengthened composites is conventionally accomplished by mechanically mixing metal powders of approximately 5 micron diameter or less with the oxide or carbide powder (preferably 0.01 micron to 0.1 micron). High speed blending techniques or conventional procedures such as ball milling may be used to mix the powder. Standard powder metallurgy techniques are then employed to form the final composite. Conventionally, however, the ceramic component is large, i.e. greater than 1 micron, due to a lack of availability, and high cost, of such small particle size materials. Further, in many cases where the particulate materials are available in the desired size, they are extremely hazardous due to their pyrophoric nature.
Alternatively, molten metal infiltration of a ceramic mat has been used to produce composites. In some cases elaborate particle coating techniques have been developed to protect ceramic particles from molten metal during molten metal infiltration. Techniques such as this have resulted in the formation of silicon carbide-aluminum composites, frequently referred to as SiC/Al, or SiC aluminum. This approach is suitable for large particulate ceramics (e.g. greater than 1 micron) and whiskers. The ceramic material, such as silicon carbide, is pressed to form a compact, and liquid metal is forced into the packed bed to fill the intersticies. Such a technique is illustrated in U.S. Pat. No. 4,444,603, of Yamatsuta et al, issued Apr. 24, 1984.
The presence of oxygen in ball-milled powders used in the prior art metallurgy techniques, or in molten metal infiltration, can result in oxide formation at the interface of the ceramic and the metal. The presence of such oxides will inhibit interfacial binding between the ceramic phase and the matrix, thus adversely effecting ductility of the composite. Such weakened interfacial contact can also result in reduced strength, loss of elongation, and crack propagation.
Internal oxidation of a metal containing a more reactive component has also been used to produce dispersion strengthened metals, such as internally oxidized aluminum in copper. For example, when a copper alloy containing about 3 percent aluminum is placed in an oxidizing atmosphere, oxygen may diffuse through the copper matrix to react with the aluminum, precipitating alumina. This technique, although limited to relatively few systems, has offered a preferred method for dispersion hardening.
In U.S. Pat. No. 2,852,366, of Jenkins, it is taught that up to 10 percent by weight of a metal complex can be incorporated into a basis metal or alloy. The patent teaches blending, pressing, and sintering a mixture of a base metal, a base metal compound of the non-metallic complexing element, and an alloy of the base metal and the complexing metal. Thus, for example, the reference teaches mixing powders of nickel, a nickel-titanium alloy, and a nickel-boron alloy, pressing, and sintering the mixed powders to form a coherent body in which a stabilizing unprecipitated "complex" of titanium and boron is dispersed in a nickel matrix. Precipitation of the complex phase is specifically avoided.
In recent years, numerous ceramics have been formed using a process referred to as self-propagating high-temperature synthesis (SHS), which involves an exothermic, self-sustaining reaction which propagates through a mixture of compressed powders. Generally the SHS process takes place at super atmospheric pressures, and is ignited by electrical impulse, thermite, or spark. The SHS process involves mixing and compacting powders of the constituent elements, and igniting the green compact with a suitable heat source. On ignition, sufficient heat is released to support a self-sustaining reaction, which permits the use of sudden, low power initiation of high temperatures, rather than bulk heating over long times at lower temperatures. Exemplary, of these techniques are the patents of Merzhanov et al. In U.S. Pat. No. 3,726,643, there is taught a method for producing high-melting refractory inorganic compound by mixing at least one metal selected from groups IV, V, and VI of the Periodic System with a non-metal such as carbon, boron, silicon, sulfur, or liquid nitrogen, and heating the surface of the mixture to produce a local temperature adequate to initiate a combustion process. In U.S. Pat. No. 4,161,512, a process is taught for preparing titanium carbide by ignition of a mixture consisting of 80-88 percent titanium and 20-12 percent carbon, resulting in an exothermic reaction of the mixture under conditions of layer-by-layer combustion. These references deal with the preparation of ceramic materials, absent the presence of a binder.
Similarly, U.S. Pat. No. 4,431,448 teaches preparation of a hard alloy by intermixing powders of titanium, boron, carbon, and a Group I-B binder metal, such as copper or silver, compression of the mixture, local ignition thereof to initiate the exothermic reaction of titanium with boron and carbon, and propagation of the ignition, resulting in an alloy comprising titanium diboride, titanium carbide, and the binder metal.
This reference, however, is limited to the use of Group I-B metals such as copper and silver, as binders, and requires local ignition. Products made by this method have low density, requiring subsequent compression and compaction.